The problem is that you gave them a wide-open invitation to your home with no specifics about your house rules or expectations and no end date for your generous accommodations.
You say you don't want to cause "friction," but certainly, it's already there.
Resentment is clearly building, and even though you've tried to keep it under wraps, it is most likely felt by your brother-in-law and his wife. Even if they are clueless and are not picking up on your frustration, if something doesn't change, you or your husband are going to blow your cool, eventually. This is no way for any of you to live.
You and your husband have to decide on an end date for these arrangements and notify the brother and his wife. If you give them 30 days to be out of your home and in their new place, that is an exceptionally generous offer, considering they've already been with you for 3 months with no expenses on their part.
Once you've notified them of the check-out date, you must also calmly, clearly let them know that they will need to help with the household and yard duties for the remainder of the stay. If they protest, say that you regret you weren't clearer from the outset, but insist, kindly and firmly, that all adults living in the house must help with the chores, no exceptions. Be prepared to list the things you want help with so that this is not left open-ended (e.g., helping with grocery shopping, meal preparation, clearing the table and doing dishes after meals; taking out the trash on specified day; dusting, vacuuming; picking up around the house).
You've tried to do a good thing here, and it was very kind of you both, but you have to consider YOUR family. It is not fair to your school-age children to have this continued disruption and no private family time with their parents. Now that your sister-in-law has a good paying job, it's time for these two adults to leave and make their own home.
Think of YOUR kids, YOUR well-being, and let them know the hotel closes in X days.